Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte solution such as Ni-MH batteries, Ni—Cd batteries, and H2SO4—Pb batteries. For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems such as ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated. Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries are solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity at low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved.
Such electrochemical devices have been produced from many companies, and the battery stability has different phases in the electrochemical devices. Accordingly, it is important to ensure the stability of the electrochemical batteries. First of all, it should be considered that errors in operation of the electrochemical device should not cause damage to users. For this purpose, the Safety Regulation strictly regulates ignition and explosion in the electrochemical devices. In the stability characteristics of the electrochemical device, overheating of the electrochemical device may cause thermal runaway, and explosion may occur when a separator is pierced. In particular, a polyolefin porous substrate commonly used as a separator of an electrochemical device shows extreme thermal shrinking behavior at a temperature of 100° C. or above due to the features of its material and its manufacturing process such as elongation, so there may occur an electric short circuit between cathode and anode.
In order to solve the above safety-related problems of the electrochemical device, Korean Laid-open Patent Publication No. 10-2006-72065 and No. 10-2007-231 disclose a separator having a porous coating layer formed by coating at least one surface of a porous substrate having many pores with a mixture of inorganic particles and a binder polymer. In the above separator, the inorganic particles in the porous coating layer formed on the porous substrate act as a kind of spacer that keeps a physical shape of the porous coating layer, so the inorganic particles restrain thermal shrinkage of the porous substrate when the electrochemical device is overheated and solve the electric short circuit problem between the cathode and the anode. In addition, interstitial volumes exist among the inorganic particles, thereby forming fine pores.
As mentioned above, the porous coating layer formed on the porous substrate attributes to the improvement of safety. In other words, in case a temperature rapidly increases due to excessive overcharging, the polyolefin porous substrate (commonly with a melt point in the range from 100 to 160° C.) having the porous coating layer attributes to the safety of a battery since pores are closed in advance. However, if the temperature of the battery increases higher, the polyolefin porous substrate may be completely melted, which may cause explosion or ignition of the battery. In addition, though the porous substrate is made of a heat-resisting material with a higher melt point than the polyolefin substrate, at excessive overcharging, temperature may rapidly increase to melt the heat-resisting substrate completely, which may cause explosion or ignition of the battery.
Meanwhile, Korean Laid-open Patent Publication No. 10-2005-66652 discloses a structure of a lithium secondary battery having different kinds of separators. In this document, a lithium ion secondary battery is an electrochemical device, which includes a plurality of unit cells, each having a first separator and a cathode and an anode positioned at both sides of the first separator; and a continuous single second separator interposed between adjacent unit cells in correspondence with each other in a laminated pattern and arranged to surround each unit cell. Here, the first and second separators adopt porous substrates made of materials with different melt points. According to this technique, though the first separator having a lower melt point is thermally shrunken, the second separator having a higher melt point is hardly thermally shrunken, thereby preventing an internal short circuit. However, the lithium secondary battery mentioned above cannot prevent a short circuit occurring in a unit cell due to thermal shrinkage of the first separator. In addition, in case temperature increases so high to cause thermal shrinkage of the second separator according to excessive overcharging, the battery may be exploded or ignited.